SBF Distances to Dwarf Elliptical Galaxies in the Sculptor Group

As part of an ongoing search for dwarf elliptical galaxies (dE) in the vicinity of the Local Group, we acquired deep B and R-band images for five dE candidates identified in the Sculptor (Scl) group region. We carried out a surface brightness fluctuation (SBF) analysis on the R-band images to measure the apparent fluctuation magnitude \bar{m}_R for each dE. Using predictions from stellar population synthesis models the galaxy distances were determined. All of these dE candidates turned out to be satellites of Scl group major members. A redshift measurement of the dE candidate ESO294-010 yielded an independent confirmation of its group membership: the [OIII] and H$_\alpha$ emission lines from a small HII region gave a heliocentric velocity of 117(\pm 5) km s-1, in close agreement with the velocity of its parent galaxy NGC 55 (v_\odot=125 km s-1). The precision of the SBF distances (5 to 10%) contributes to delineating the cigar-like distribution of the Scl group members, which extend over distances from 1.7 to 4.4 Mpc and are concentrated in three, possibly four subclumps. The Hubble diagram for nine Scl galaxies, including two of our dEs, exhibits a tight linear velocity--distance relation with a steep slope of 119 km s-1 Mpc-1. The results indicate that gravitational interaction among the Scl group members plays only a minor role in the dynamics of the group. However, the Hubble flow of the entire system appears strongly disturbed by the large masses of our Galaxy and M31 leading to the observed shearing motion. From the distances and velocities of 49 galaxies located in the Local Group and towards the Scl group, we illustrate the continuity of the galaxy distribution which strongly supports the view that the two groups form a single supergalactic structure.Comment: To appear in The Astronomical Journal, December 1998; 28 pages with 22 figure

More evidence for hidden spiral and bar features in bright early-type dwarf galaxies

Following the discovery of spiral structure in IC3328 (Jerjen et al.~2000), we present further evidence that a sizable fraction of bright early-type dwarfs in the Virgo cluster are genuine disk galaxies, or are hosting a disk component. Among a sample of 23 nucleated dwarf ellipticals and dS0s observed with the Very Large Telescope in $B$ and $R$, we found another four systems exhibiting non-axisymmetric structures, such as a bar and/or spiral arms, indicative of a disk (IC0783, IC3349, NGC4431, IC3468). Particularly remarkable are the two-armed spiral pattern in IC0783 and the bar and trailing arms in NGC4431. For both galaxies the disk nature has recently been confirmed by a rotation velocity measurement (Simien & Prugniel 2002). Our photometric search is based on a Fourier decomposition method and a specific version of unsharp masking. Some ``early-type'' dwarfs in the Virgo cluster seem to be former late-type galaxies which were transformed to early-type morphology, e.g. by ``harassment'', during their infall to the cluster, while maintaining part of their disk structure.Comment: A&A accepte

Statistical analysis of bound companions in the Coma cluster

The rich and nearby Coma cluster of galaxies is known to have substructure. We aim to create a more detailed picture of this substructure by searching directly for bound companions around individual giant members. We have used two catalogs of Coma galaxies, one covering the cluster core for a detailed morphological analysis, another covering the outskirts. The separation limit between possible companions (secondaries) and giants (primaries) is chosen as M_B = -19 and M_R = -20, respectively for the two catalogs. We have created pseudo-clusters by shuffling positions or velocities of the primaries and search for significant over-densities of possible companions around giants by comparison with the data. This method was developed and applied first to the Virgo cluster by Ferguson (1992). In a second approach we introduced a modified nearest neighbor analysis using several interaction parameters for all galaxies. We find evidence for some excesses due to possible companions for both catalogs. Satellites are typically found among the faintest dwarfs (M_B < -16) around high-luminosity primaries. The most significant excesses are found around very luminous late-type giants (spirals) in the outskirts, which is expected in an infall scenario of cluster evolution. A rough estimate for an upper limit of bound galaxies within Coma is 2 - 4 percent, to be compared with ca. 7 percent for Virgo. The results agree well with the expected low frequency of bound companions in a regular cluster such as Coma.Comment: Astronomy & Astrophysics, in press; 17 pages, 13 figure

Virgo Early-Type Dwarfs in ALFALFA

Early-type dwarf galaxies dominate cluster populations, but their formation and evolutionary histories are poorly understood. The ALFALFA (Arecibo Legacy Fast ALFA) survey has completed observations of the Virgo Cluster in the declination range of 6 - 16 degrees. Less than 2% of the early-type dwarf population is detected, a significantly lower fraction than reported in previous papers based on more limited samples. In contrast ~30 of the irregular/BCD dwarf population is detected. The detected early-type galaxies tend to be located in the outer regions of the cluster, with a concentration in the direction of the M Cloud. Many show evidence for ongoing/recent star formation. Galaxies such as these may be undergoing morphological transition due to cluster environmental effects.Comment: 2 pages. To be published in proceedings of IAU Symposium 244: 'Dark Galaxies and Lost Baryons', J. I. Davies & M. D. Disney. eds., Cambridge University Pres

HI Clouds detected towards Virgo with the Arecibo Legacy Fast ALFA Survey

The Arecibo Legacy Fast ALFA survey is in the process of yielding a complete HI dataset of the Virgo Cluster and its environs (Giovanelli et al. 2007, Kent et al., in preparation). Assuming a distance to Virgo of 16.7 Mpc, the minimum detectable HI mass by ALFALFA is of order 2 x 10^7 Msun. A number of the HI detections appear to have interesting properties. Some appear associated with, but offset from, low surface brightness optical counterparts; others, at larger spatial offsets, may be tidally related to optical counterparts. Yet another class includes detections which are not identifiable with any optical counterparts. We present the ALFALFA results on these objects in the Virgo region, as well as followup aperture synthesis observations obtained with the VLA.Comment: To appear in Proceedings of IAU Symp #244, "Dark Galaxies and Lost Baryons", June 2007, 10 pages including 7 figures and 1 tabl

The stellar correlation function from SDSS - A statistical search for wide binary stars

We study the statistical properties of the wide binary population in the Galaxy field with projected separations larger than 200 AU by constructing the stellar angular two-point correlation function (2PCF) from a homogeneous sample of nearly 670'000 main sequence stars. The selected stars lie within a rectangular region around the Northern Galactic Pole and have apparent r-band magnitudes between 15 and 20.5 mag and spectral classes later than G5 (g-r > 0.5 mag). The data were taken from the Sixth Data Release of the Sloan Digital Sky Survey. We model the 2PCF by means of the Wasserman-Weinberg technique including several assumptions on the distribution of the binaries' orbital parameters, luminosity function, and density distribution in the Galaxy. In particular, we assume that the semi-major axis distribution is described by a single powerlaw. The free model parameters - the local wide binary number density and the power-law index of the semi-major axis distribution - are inferred simultaneously by least-square fitting. We find the separation distribution to follow Oepik's law up to the Galactic tidal limit, without any break and a local density of 5 wide binaries per 1'000 cubic parsec with both components having spectral type later than G5. This implies that about 10% of all stars in the solar neighbourhood are members of such a late-type wide binary system. With a relative statistical (2 sigma) error of about 10%, our findings are in general agreement with previous studies of wide binaries. The data suggest that about 800 very wide pairs with projected separations larger than 0.1 pc exist in our sample, whereas none are found beyond 0.8 pc.Comment: 18 pages, 14 figures, 7 tables; added reference for section

Orbital ordering, Jahn-Teller distortion, and resonant x-ray scattering in KCuF3

The orbital, lattice, and spin ordering phenomena in KCuF3 are investigated by means of LDA+U calculations, based on ab-initio pseudopotentials.We examine the Cu-3d orbital ordering and the associated Jahn-Teller distortion in several different spin-ordered structures of KCuF3. The ground state is correctly predicted to be an A-type antiferromagnetic structure, and the calculated Jahn-Teller distortion agrees also well with experiment. Concerning the orbital ordering, we find that even for a highly ionic compound such as KCuF3, the orbital-order parameter is significantly reduced with respect to its nominal value due to Cu(3d)--F(2p) hybridization. We also calculate the Cu K-edge resonant x-ray scattering spectra for Bragg reflections associated with orbital order. Consistent with previous studies, we find that the resonant signal is dominated by the structural anisotropy in the distribution of the F neighbors of the resonant Cu atom, and that the Cu-3d orbital ordering has only a minor influence on the spectra. Our LDA+U results, however, also indicate that a change in the magnetic structure has a small influence on the Jahn-Teller distortion, and hence on the resonant spectrum, in the conventional (room-temperature) crystallographic structure of KCuF3. This may indicate that the large change observed experimentally in the resonant signal near the N\'eel temperature is related to a low-temperature structural transformation in KCuF3.Comment: 11 pages, 9 figure

New low surface brightness dwarf galaxies in the Centaurus group

We conducted an extensive CCD search for faint, unresolved dwarf galaxies of very low surface brightness in the whole Centaurus group region encompassing the Cen A and M 83 subgroups lying at a distance of roughly 4 and 5 Mpc, respectively. The aim is to significantly increase the sample of known Centaurus group members down to a fainter level of completeness, serving as a basis for future studies of the 3D structure of the group. Following our previous survey of 60 square degrees covering the M 83 subgroup, we extended and completed our survey of the Centaurus group region by imaging another 500 square degrees area in the g and r bands with the wide-field Dark Energy Survey Camera at the 4m Blanco telescope at CTIO. The limiting central surface brightness reached for suspected Centaurus members is $\mu_r \approx 29$ mag arcsec$^{-2}$, corresponding to an absolute magnitude $M_r \approx -9.5$. The images were enhanced using different filtering techniques. We found 41 new dwarf galaxy candidates, which together with the previously discovered 16 dwarf candidates in the M 83 subgroup amounts to almost a doubling of the number of known galaxies in the Centaurus complex, if the candidates are confirmed. We carried out surface photometry in g and r, and report the photometric parameters derived therefrom, for all new candidates as well as previously known members in the surveyed area. The photometric properties of the candidates, when compared to those of LG dwarfs and previously known Centaurus dwarfs, suggest membership in the Centaurus group. The sky distribution of the new objects is generally following a common envelope around the Cen A and M 83 subgroups. How the new dwarfs are connected to the intriguing double-planar feature recently reported by Tully et al. (2015) must await distance information for the candidates.Comment: 14 pages, 12 figures, accepted for publication in A&